Carbon is found in nature in several allotropic forms. Two well known allotropes of carbon are diamond and graphite. Diamond has a cubic structure formed by a tetrahedral laffice of sp3-bonded carbon atoms. This structure has great strength and makes diamond a hard material. Graphite is composed of sp2-hybridized carbon atoms bonded together in layers. Individual layers are stacked to form bulk graphite, which is quite soft; the layers are not strongly associated and so are able to slip relative to one another. This slipping of layers is generally believed to be the source of the lubricating qualities of graphite.
Other well-known forms of carbon include soot, charcoal, and other amorphous materials, which are all composed of finely divided graphite. Graphite is also known to exist as whiskers or fibers which have high tensile strength.
More recently, a third allotrope of carbon, the fullerenes, has been discovered and extensively studied (see e.g., "fullerenes" Scient. Am., October 1991, pp. 54-63). The fullerenes consist of sp2-hybridized carbons bonded into a closed shell. The simplest and most studied fullerene has 60 carbon atoms arranged in the form of a truncated icosahedron; the shape of this molecule resembles a soccer ball, and it has both 5- and 6-membered rings in its structure. Other fullerenes having 70, 84, and higher numbers of carbon atoms have also been characterized. A related structure is the carbon nanotube, which consists of carbon atoms arranged so as to form a cylinder.
The known allotropes of carbon are based either on sp3 (diamond) or sp2 (graphite, fullerenes, nanotubes) hybridization of carbon. The existence of other allotropes have been mentioned previously, however, the acetylenic carbon allotrope was generally held to be unstable. Although there were papers from the former Soviet Union in the 50s and 60s claimimg that a new form of carbon called "carbyne" had been prepared (Mel'nichenko, V. M. et al., Carbon 21, 131 (1983), and references contained therein.), other workers have extensively studied their evidence and this contention is generally held to be incorrect (see e.g. P. P. K. Smith and P. R. Buseck, Science 216, 984 (1982); J. Jansta, F. P. Dousek, V. Patazelova, Carbon 13, 377 (1975); W. A. Little, Phys. Rev. 134, 1416 (1964); M. F. Hawthorne, Preliminary Reports, Memoranda and Technical Notes of the Materials Research Council Summer Conference, La Jolla, Calif., July 1973 (NTIS) ).
The synthesis of long-chain acetylenic carbon species [(.alpha.-.omega.-bis(triethylsilyl)polyynes up to 32 carbon atoms] with alternating single and triple bonds was reported by David Walton and co-workers in 1972 (R. Eastmond, T. R. Johnston, D. R. M. Walton, Tetrahedron 28, 4601 (1972).). They prepared, using copper chloride (Hay coupling), mixtures of acetylenic carbon compounds that contained 2-16 acetylene units. Walton reported that beyond eight carbon atoms the chains became increasingly unstable with increasing length, and only the lowest members of the series were isolable.
Diederich and co-workers (F. Diederich et al, Science 245, 1088 (1989) ) have reported the synthesis of mass spectrometric quantities of a C.sub.18 alkyne-containing carbon ring by flash heating [18] annulene precursors. Diederich has also reported the synthesis of acetylenic carbon species containing up to 6 acetylene units (F. Diederich et al., J. Am. Chem. Soc. 113, 6943 (1991) ).